Doping and Surface Effects of CuFeS₂ Nanocrystals Used in Thermoelectric Nanocomposites

Abstract

In the search for low‐cost thermoelectric materials operating near room temperature, the potential of chalcopyrite (CuFeS2) nanocrystals is explored. Their colloidal synthesis is optimized to achieve around 40 nm sized nanocrystals with the goal to effectively reduce thermal conductivity via phonon scattering while maintaining high electrical conductivity. EDX and XPS analyses reveal that the nanocrystals are intrinsically nanostructured with a radial compositional gradient. Three strategies are explored to optimize the thermoelectric properties: i) Intrinsic doping by varying the Cu : Fe ratio. However, the effect of this variation is overcompensated by a global sulfur deficiency, making the chalcopyrite nanocrystals n‐type. A high Seebeck coefficient, S, up to −380 μV/K is obtained, while the figure of merit remains comparably low (ZT=0.07 at 400 °C) because of low electrical conductivity σ. ii) Removal of the native, insulating ligands by exchange with potassium selenide. This results in a better trade‐off between S and σ and hence a strongly improved ZT (0.18 at 400 °C). iii) Extrinsic doping via intimate mixture of chalcopyrite nanocrystals with metal nanoparticles. Sn (3 wt %) or Ag (16 wt %) nanoparticles give the best results (ZT=0.16 at 400 °C), inducing the concomitant reduction of thermal conductivity κ and increase of σ.

Mechanochemistry for Energy Materials: Impact of High‐Energy Milling on Chemical, Electric and Thermal Transport Properties of Chalcopyrite CuFeS2 Nanoparticles

“…To help in elucidation of solid‐state changes in CuFeS 2 , EDX analysis of the powdered samples was performed (Table 1 ). EDX method has penetration depth ∼150 nm [7h] and can provide the average content of elements in bulk of the synthesized species for which the average crystallite size ∼12 nm was determined by XRD in our case. The EDX data for copper and iron content in solid phase show some variation of the values.…”

Thermoelectric Cu–S-Based Materials Synthesized via a Scalable Mechanochemical Process

“…As the n-type doping procedure, several approaches can be used; among all, we note (i) the Fe overstoichiometry in Cu 1– x Fe 1+ x S 2 or the Cu 1+ substitution by Tm 2+ species (Zn, Pd, ...) in Cu 1– x Tm x FeS 2 leading to the generation of conducting electrons due to compensation of excess positive charge on Cu sites and (ii) sulfur deficiency CuFeS 2– x , where the S deficiency, linked with sulfur vaporization during synthesis/compaction process, suppresses lattice thermal conductivity and acts as donor doping. We underline that, in the case of chalcopyrite, which is semiconducting in nature, the optimized doping must be used to improve thermoelectric performance with higher ZT values. − …”

“…In this respect the chalcopyrite has been reported as an interesting n-type thermoelectric material with a reported value of ZT=0.2 35 . As n-type doping procedure several approaches can be used; among all we note (i) the Fe overstoichiometry in Cu We underline that in case of chalcopyrite, which is semiconducting in nature, the optimized doping must be used to improve thermoelectric performance with higher ZT values [36][37][38][39][40][41] .…”

Synthesis, Bottom up Assembly and Thermoelectric Properties of Sb-Doped PbS Nanocrystal Building Blocks

“…However, introducing controlled amounts of electronic dopants into colloidal nanocrystals (NCs) has been often a main challenge. A particular application, where such electronic doping is key and where the use of nanomaterials presents a clear advantage, is thermoelectricity [ 3 , 4 , 5 , 6 ]. All relevant thermoelectric properties, electrical conductivity ( σ ), Seebeck coefficient ( S ) and thermal conductivity ( κ ), strongly depend on charge carrier concentration.…”